Biofilm Removal Processes

Biofilm removal processes involve the actual loss of material from the biofilm—either cells or cell-biofilm matrix debris. Desorption (see "Deposition Processes") is the release of cells from the substratum and occurs during the early stages of substratum colonization. Biofilm removal represents loss of biomass from a developing or mature biofilm that arises by way of two distinctively different mechanisms: detachment and sloughing.

Detachment. Detachment is the continuous loss of biomass or individual cells from a biofilm. The rate of biofilm detachment has been mathematically modeled in a number of ways (61). In about the only definitive experimental study on biofilm detachment, Applegate and Bryers (62) reported that the growth conditions of the biofilm strongly affect biofilm removal processes. Carbon-substrate-limited growth conditions fostered biofilms that (1) contained less extracellular polymer per cell and per area, (2) bound less calcium per unit amount of biofilm, and (3) exhibited a higher detachment rate than oxygen-substrate-limited biofilms that accumulated more extracellular polymer per cell, bound more calcium per biofilm amount, and developed a rigid compact morphology that was very resistant to detachment prior to predictable sloughing. Carbon-limited biofilms did not exhibit sloughing for extended pe riods of cultivation or after prolonged (300 h) of carbon-nutrient deprivation.

Perhaps biofilm detachment processes may be controlled, not by fluid hydrodynamics as once imagined, but rather by cell physiology. Boyd and Chakrabarty (63) suggest biofilm detachment could be genetically controlled. Their work showed that the exopolysaccharide alginate of P. aeruginosa was important in determining the degree of cell detachment from an agar surface. Nonmucold strain 8822 gave rise to 50-fold more sloughed cells than mucoid strains 8821 and 8830. Alginate anchors the bacteria to the surface, thereby influencing the extent of detachment. The role of the P. aeruginosa alginate lyase in the process of cell detachment was investigated. Increased expression of the alginate lyase in mucoid strain 8830 led to alginate degradation and increased cell detachment. Similar effects were seen both when alginate lyase synthesis was induced at the initial stage of cell inoculation and when it was induced at a later stage of growth. Apparently, high molecular weight alginate polymers are required to efficiently retain the bacteria within the developing biofilm. When expressed from a regulated promoter, the alginate lyase did enhance detachment of cells through the degradation of the alginate. This suggests a possible role for the lyase in the removal process of bacterial growth films.

Sloughing. The catastrophic, apparently random, loss of large pieces or entire sections of biofilm is known as sloughing (Fig. 5). At the industrial scale, sloughing creates excursions of intolerably high biomass in bioreactor effluents, and periodic "blooms" of pathogenic bacteria in drinking water distribution systems. At the laboratory scale, all too often, random sloughing of a biofilm will terminate an experiment, resulting in very little quantitative data regarding the fundamental causes of sloughing. A number of situation-specific causes of sloughing have been identified: bubble formation in either anaerobic methane-producing biofilms or denitrifying biofilms, and artificially induced sloughing by way of EGTA addition, which che-lates the bound calcium ion used in biofilm alginate cross-linking. Applegate and Bryers (62) reports that the growth conditions of the biofilm strongly affected the biofilm removal processes of detachment and sloughing. "Fluffier" biofilms, produced under carbon-limited growth, exhibited high detachment rates but never sloughed, even when subjected to over 300 h of nutrient starvation. Conversely, rigid biofilms cultivated under oxygen limitations showed little detachment, but the onset of catastrophic sloughing was repeatable between replicate experiments.

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